In the early history of studies on adaptations in plants to habitat constraints, correlations were made on a rather empirical basis. Plants found growing in dry conditions for instance have been studied and shown to exhibit anatom-ical modifications not normally associated with plants from more mesic lo-calities. Without any attempt at experimentation, the authors of that period would ascribe specific properties to the structures they saw. For example, Haberlandt’s book, Physiological Plant Anatomy was written largely from observation with little or no experimentation, and must therefore be used with care. Many researchers subsequent to Haberlandt have adopted his ideas uncritically. Where people have taken the trouble to study the anatomy of a range of plants from one habitat, they have found some fea-tures which seem to vary so widely in expression, for example in thickness of the walls of epidermal cells, that their adaptive significance is put in doubt. There are, however, certain types of modification which crop up with such a degree of regularity, and in such taxonomically diverse plants, that they might really be related to survival in that particular habi-tat. Such features lend themselves to experimental studies in physiology and studies on genes that control both the development of the structure and its function.
Despite any adaptations found in the anatomy of plants which might be thought to be of ‘ecological’ benefit, it is normal for family or generic char-acters to be well expressed and often dominant.
Not all adaptations are evident at the anatomical and morphological lev-els. Some are physiological, and physiological races of plants have evolved which fit them for growth in extreme conditions. For example, some races of Agrostis species can grow in areas of high concentration of heavy metals (e.g. copper) where other plants fail. These adapted grasses have been shown to accumulate and immobilize heavy metals in their roots, pre-venting these metals from entering and damaging cells and organelles in other organs.
The duration of life of the plant might be a dominant feature, which helps a species to survive. Ephemeral species may grow in normally xeric condi-tions if they can germinate their seeds, grow, flower and fruit when water is available. During this short period of activity, the plant may have adequate water and would not need any other xeromorphic adaptations.
The issue is made more complex when it is realized that there are often many microecological niches even within a small area. Diversity in anatomy could relate to such differences, which are often hard to detect without prolonged study of the area concerned. Seasonal variability in en-vironment may be overlooked by those making plant collections at particu-lar times of the year. It boils down to the observation that if a species is found growing successfully under a given set of conditions, it is there as a result of selection, adaptation and ability to compete with other species for that niche.
Some of the main habitats and commonly associated plant modifications are outlined below. Despite the cautionary remarks that we have expressed above, it is often possible to find in plants anatomical features which do show a close correlation to the habitat type in which they normally occur and which are clearly a result of adaptation to special conditions and physio-logical needs.